Today it is common for people to be concerned about the amount of fat in their diets. This is because we eat too much of it not because it is harmful in all quantities.
I will use the terms fat and oil interchangeably. Fats have higher melting points than those that generally remain liquid at room temperature that are called oils. Other than that they have very similar molecular structures and uses. I am not going into saturated, unsaturated, trans fats as those distinctions are not important to cooking and would need another story.
Here is my take on the functions of fats in cooking. For those who glazed over in science class I will keep it simple but understanding the reason why will help your cooking.
We all know about pan frying. To stop food sticking you need a film of oil. The film can be quite thin but even with the best non-stick surface you will struggle to avoid adding any oil at all unless the food itself produces it. This allows higher temperatures than water cooking which adds browning and speeds up the process.
We often think of fried food as cheap and not very healthy but it isn’t necessarily. Stir frying can produce lean tasty food using very little oil. Frying is also the first step in braising and exchange cooking which are important methods in many cuisines.
Fats have different smoke points – the temperature where they start to break down. The fat that you fry with should be selected according to how hot you want the pan to be as if it burns the flavour is not always attractive. You can easily find tables of fats and their properties.
Deep frying is a way of cooking food quickly at high temperature. It allows the food to heat very quickly by immersing it so that the heat penetrates from all sides. This speed is import to the flavour and texture of some dishes and is hard to get any other way. It is inevitable that any crumb or batter coating will absorb some fat.
The modern air fryer tries to emulate quick immersion cooking but in many cases it only reheats food that was deep fried beforehand. Even if you make hot air flow past food quickly with a fan direct contact with hot oil transfers heat faster.
Shallow frying involves heat transfer too. Try toasting cashews in a dry pan and with some oil – you will get quite different results.
Fats provide an important aspect of texture and mouth feel. You cannot make good sausages without a reasonable amount of fat as they will be dry in the mouth. Cheese and chocolate would not feel the same in the mouth without fats. The price of chocolate is linked to the skill of the maker in getting that silky texture we love. Steak and even the humble hamburger rely on fat to some degree for attractive texture
We prize some fats for their own flavour. Butter, cream, olive oil, coconut oil and others have distinctive flavours that we like to incorporate into food. In other cases we use neutral flavoured fats as we want the flavour of other ingredients to come through.
Browning is an almost universal way of adding flavour to food used in every cuisine. It is the result of Maillard reactions where more complex flavour compounds are made from simpler ones by the application of heat. You can do it at lower temperatures but it takes a long time and you can do it by dry heat, as in the oven or flame grilling, but very often we do it by frying. The reason browning meat improves flavour is not because it seals in the juices but because of Maillard Browning.
Many flavour substances are soluble in water, for example; sugars, salts, acids, broken down proteins etc, however there are many substances that are important for flavour and aroma that are not very soluble in water.
For example, the flavour compounds in herbs and spices (usually termed essential oils) are mainly fat soluble. There are many dishes that begin by frying spices with other ingredients like garlic, ginger onion etc. Part of the reason is that heat changes the nature of spices - although this can be done by dry roasting too with somewhat different results. Another important reason is to extract the flavour compounds. In this role regardless of fat adding its own flavour it dissolves and carries the flavour of water insoluble substances.
Braising starts with frying and then you add water or water-based liquids. This way the best of both water soluble and insoluble flavours can be incorporated. Sometimes the liquid is bound by an emulsifying agent (flour, tomato paste, etc). Other times the process continues with exchange cooking, such as curry or stew, where the main food ingredient adds flavour to the broth and the flavours in the broth are absorbed by the main ingredient. You cannot do all that without any fat.
We actually need some fat in our diets. To remain healthy you can have a low fat diet but not a no fat diet. We love to eat some foods like sugar and fat because we have evolved to enjoy them as they were (before these days of endemic obesity) significant sources of necessary energy. Today we mostly don’t need to consume so much high energy food but we still need some fat.
Fats are important as carriers of fat-soluble vitamins and the precursors to some important body chemicals that we cannot make. Just as there are essential vitamins and proteins that must be ingested there are essential fats.
Don’t think that I want people to eat more fat but it does have an important role in food and trying to eliminate fat is hardly possible. Certainly limit your energy intake to a healthy level but do it without thought and food will be bland and less interesting. It is possible to maintain variety and balance pleasure and health. If you understand why food processes work you will be a better cook.